In low tension distribution ignition systems, there is one ignition coil per spark plug. In a typical application, these systems are required to operate on an essentially continuous basis with minimal down time. While the incorporation of diagnostic functions into ignition systems has become commonplace, these have generally been directed to system functions on a global basis, that is, magnetic pickup signals, programming errors and failed components. Products are offered which provide a diagnostic message on an individual cylinder basis for open coil primary faults. Since the ignition system components most often requiring service or replacement are on the secondary side of the ignition coil, that is, the spark plug, the secondary wire, the insulating boots, etc., the ability to monitor the secondary side is highly desirable. Unfortunately, existing means of monitoring and analyzing the performance and/or condition of these components is either costly or impractical and may require an oscilloscope with high voltage probe, a pulse transformer on the secondary wire, a flame ionization detector in the cylinder and so forth. It is an advantage, according to this invention, that it requires none of the above equipment and is relatively low in cost. The secondary (and primary) diagnostic function uses the measured voltage of the storage capacitor to capture discharge time to detect the presence of certain conditions requiring attention and generates the appropriate diagnostic messages.
U.S. Pat. No. 5,208,540 entitled "Ignition Performance Monitor and Monitoring Method for Capacitive Discharge Ignition Systems" uses electrical current sensing on the primary side of a low tension capacitive discharge ignition system to determine the time period during which current generated in the primary winding takes to decay to zero and uses this to calculate and indicate the firing voltage to fire a given spark plug. However, this patent makes no disclosure for a means to detect an open primary, a primary shortage, a capacitor not charged, an open secondary or a shorted secondary.